Efficient Removal of Heavy Metal Ions with Biopolymer Template

Nov 30, 2011 - Effective Removal of Heavy Metal Ions from Industrial Wastes Using ..... Mesoporous titania spheres derived from sodium alginate-gum ac...
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Efficient Removal of Heavy Metal Ions with Biopolymer Template Synthesized Mesoporous Titania Beads of Hundreds of Micrometers Size Na Wu, Huanhuan Wei, and Lizhi Zhang* Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China S Supporting Information *

ABSTRACT: We demonstrated that mesoporous titania beads of uniform size (about 450 μm) and high surface area could be synthesized via an alginate biopolymer template method. These mesoporous titania beads could efficiently remove Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions from simulated wastewater with a facile subsequent solid−liquid separation because of their large sizes. We chose Cr(VI) removal as the case study and found that each gram of these titania beads could remove 6.7 mg of Cr(VI) from simulated wastewater containing 8.0 mg·L−1 of Cr(VI) at pH = 2.0. The Cr(VI) removal process was found to obey the Langmuir adsorption model and its kinetics followed pseudo-second-order rate equation. The Cr(VI) removal mechanism of titania beads might be attributed to the electrostatic adsorption of Cr(VI) ions in the form of negatively charged HCrO4− by positively charged TiO2 beads, accompanying partial reduction of Cr(VI) to Cr(III) by the reductive surface hydroxyl groups on the titania beads. The used titania beads could be recovered with 0.1 mol·L−1 of NaOH solution. This study provides a promising micro/nanostructured adsorbent with easy solid−liquid separation property for heavy metal ions removal.

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nanoparticles,17 and amino-functionalized Fe3O4@SiO2 core− shell magnetic nanomaterial,18 were used to remove heavy metal ions from simulated wastewater. Besides magnetic nanoadsorbents, hierarchical adsorbents with micro/nanostructure could be also easily separated from liquid phase because of their micrometer scale sizes. Compared to magnetic adsorbents, micro/nanostructured adsorbents possess much higher surface areas, and thus are more attractive. For example, monodisperse mesoporous titania beads with controllable diameter, high surface areas, and variable pore diameters were synthesized through a sol−gel and solvothermal process in the presence of hexadecylamine as a structured-directing agent.19 Hierarchical micro/nanostructured ceria20,21 and copper oxide22 could be prepared via an alcohol-mediated synthesis process with or without a template. Y2O3 hierarchical structure with high surface areas was obtained by a simple sonochemical method.23 Among these micro/nanostructured oxides, CeO2, CuO, and Y2O3 were found to possess high removal efficiency for heavy

INTRODUCTION Toxic heavy metal ions in water bring many detrimental effects on environment and human health.1−3 Chemical precipitation,4 reverse osmosis,5 electrochemical treatment techniques,6 ion exchange,7,8 membrane filtration,9 and adsorption10 have been used for the removal of heavy metal ions from wastewater. Among them, adsorption is found to be a promising technique to remove heavy metal ions from effluents because of its low operational and maintenance costs and high efficiency, especially for the heavy metal ions with low concentration. Nanostructured adsorbents exhibited remarkable advantages owing to their higher surface areas and much more surface active sites than bulk materials.11,12 Unfortunately, it is very difficult to separate them from the wastewater because of their high surface energy and nanosize, which restricts their applications. Therefore, it is of great importance to develop nanostructured adsorbents with easy solid−liquid separation property. For this purpose, many researchers turn to designing magnetic nanoadsorbents because of their easy separation via external magnetic fields. For instance, magnetic adsorbents, including surface-modified MnFe2O4,13 magnetic hydroxyapatite nanoparticles,14 Fe@Fe2O3 core−shell nanowires,15 magnetic chitosan nanocomposites,16 chitosan-bounded Fe3O4 © 2011 American Chemical Society

Received: Revised: Accepted: Published: 419

June 15, 2011 October 27, 2011 November 30, 2011 November 30, 2011 dx.doi.org/10.1021/es202043u | Environ. Sci. Technol. 2012, 46, 419−425

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Figure 1. Schematic illustration of the biopolymer template synthesis of mesoporous titania beads.

area and porosity analyzer at 77 K after the sample had been degassed in the flow of N2 at 180 °C for 5 h. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh-vacuum (UHV) chambers. All the binding energies were calibrated to the C 1s peak at 284.5 eV of the surface adventitious carbon. Heavy Metal Ions Removal Experiments. Chromium ions sorption experiments were performed according to the batch method. Simulated wastewater with different Cr(VI) concentrations (2.0, 4.0, 6.0, 8.0, 10.0, 15.0 mg·L−1) were prepared by dilution of the stock K2Cr2O7 standard solution with DI water. Titania beads (0.10 g) were added to 100 mL of the above Cr(VI) solution under mechanical agitation. For all adsorption tests, the initial pH values of the Cr(VI) solutions were adjusted with 0.1 mol·L−1 HNO3 solution or 0.1 mol·L−1 NaOH solution. After the adsorption processes, the beads could be conveniently separated by decantation and the supernatant was immediately analyzed by atomic absorption spectrometry (WFX-1F2, China). To study the influence of initial pH on the removal of Cr(VI), the initial pH values of the solutions were adjusted to 2, 4, 6, and 9. The concentration of titania beads was 1.0 g·L−1. The initial Cr(VI) concentration was 8.0 mg·L−1. For the regeneration, the Cr(VI)-adsorbed titania beads were immerged in 5 mL of NaOH solution (0.1 mol·L−1) for 5 h and then washed five times with DI water to remove adsorbed alkali. The removal experiments of Cd(II), Cr(III), Cu(II), and Co(II) ions with initial concentration of 10.0 mg·L−1 were similar to that of Cr(VI), but the initial pH values of the solutions containing Cd(II), Cr(III), Cu(II), and Co(II) ions were not adjusted before the removal experiments. The initial pHs for Cd(II), Cr(III), Cu(II), and Co(II) solutions were 5.9, 4.2, 5.6, and 5.9, respectively.

metal ions removal from aqueous solution and easy solid− liquid separation property. In this study, we utilized a modified alginate biopolymer template method24 to synthesize mesoporous titania beads with high surface area and uniform size (about 450 μm). The asprepared titania beads were used to remove heavy metal ions from simulated wastewater. The adsorption kinetics and the possible Cr(VI) removal mechanism of titania beads as well as the recovery of used titania beads were investigated in detail.

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EXPERIMENTAL SECTION Materials. Sodium alginate powder, calcium chloride, ethanol, and isopropanol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All these chemicals were analytical grade and used as received without further purification. Titanium(IV) isopropoxide (TTIP, 97%) was purchased from Aldrich. DI water was used throughout the experiments. Biopolymer Template Synthesis of Titania Beads. A schematic diagram of the synthesis procedure is shown in Figure 1. First, the calcium alginate hydrogel microspheres were prepared by adding 1 wt % of sodium alginate solution into 0.1 mg·L−1 of calcium chloride solution drop by drop with a 1-mL syringe. The resulting hydrogel microspheres were soaked in a series of water/ethanol composite solutions with volume ratios ranging from 2:1, 1:2, to 0:1, in sequence, and finally in pure isopropanol. The duration of each soaking was 6 h. The obtained calcium alginate alcogel microspheres were immersed in 30 wt % TTIP/isopropanol solution for 18 h and then transferred to a water/isopropanol solution with volume ratio of 1:1 for hydrolysis and mineralization at 25 °C for 6 h. The hydrolyzed product was anatase titania (Figure S1, Supporting Information). The white hybrid calcium alginate/titania beads were dried at room temperature, then calcined at 450 °C for 4 h in air to remove the alginate and increase the crystallinity of the titania. The rate of heating was 3 °C·min−1. Finally, the beads were washed with 0.1 mol·L−1 HNO3 solution and DI water to remove CaCO3 to obtain titania beads. Characterization of the Beads. The structure and morphology of the final beads were determined by X-ray diffraction (XRD, Rigaku Ultima III), optical microscopy (OM, Leica DMI3000 B), scanning electron microscopy (SEM, JEOL 6700-F), and transmission electron microscopy (TEM, JEOL JSM-2010). The sample for transmission electron microscopy (TEM) was prepared by dispersing the ground titania beads powder in ethanol and the dispersion was dropped on a carbon−copper grid. Nitrogen adsorption−desorption isotherm was collected on a Micromeritics Tristar-3000 surface

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RESULTS AND DISCUSSION Sample Characterization. Typical OM image and SEM images of the final beads are shown in Figure 2. The OM image (Figure 2a) reveals that the beads are of spherical shape with relatively uniform size of about 450 μm. SEM characterization confirms the spherical shape of the beads (Figure 2b) and reveals that these beads possess rough surface and numerous pores (Figure 2c). SEM image of ground sample displays that the primary titania nanoparticles are several nanometers in size (Figure 2d). The XRD pattern of the final beads could be well indexed to anatase titania (JCPDS, file 21-1272) with good crystallinity (Figure 2e). According to the Debye−Scherrer formula, the average crystallite size of the resulting titania was 420

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Figure 2. (a) OM and (b) SEM images of titania beads; (c) SEM image of the titania beads surface; (d) SEM image of ground titania beads; (e) XRD pattern of titania beads; (f) nitrogen adsorption−desorption isotherm and Barrett−Joyner−Halenda (BJH) pore size distribution (inset) of the final titania beads.

estimated to be 8.5 nm. The TEM image of titania nanoparticles confirmed that the size of titania nanoparticles was about 8 nm (Figure S2, Supporting Information), consistent with XRD result. The porous structure and specific surface area of the final titania beads were studied by nitrogen adsorption−desorption experiment (Figure 2f). The observed type IV isotherm with an H2-type hysteresis loop at relative pressure of P/P0 = 0.43−0.85 illuminates the presence of interconnected mesopores in the final titania beads.25 Moreover, the pore size in the final titania beads ranges from 2 to 6 nm (inset of Figure 2f). There might be two origins for the mesoporous structure of the titania beads. One is via the burning of the alginate biopolymer template, leaving behind the mesoporous structure. The other is via the agglomeration of titania nanocrystals.26 The BET specific surface area of the final titania beads is 144 m2·g−1, significantly higher than that (89 to 120 m2·g−1) of mesoporous titania beads synthesized through a sol−gel and solvothermal process in the presence of hexadecylamine as a structure-directing agent.19 We therefore conclude that mesoporous titania beads with high surface area and large size could be prepared by the alginate biopolymer template method. Removal of Heavy Metal Ions. The titania beads were used to remove heavy metal ions from simulated wastewater. The adsorption capacities of Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions were 9.39, 8.94, 8.93, 8.40, and 7.62 mg·g−1, respectively. The equilibrium times of different heavy metal ions adsorbed on titania beads were within 120 min. More importantly, more than 55% of heavy metal ions could be removed by titania beads within 60 min (Figure 3). The metal ions adsorption kinetics was found to fit a pseudo-second-order model (eq 1).27

t 1 1 = + t 2 qt qe k 2qe −1

Figure 3. Time profile of Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions removal with titania beads. The concentration of titania beads was 1.0 g·L−1. The initial concentration of Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions was 10 mg·L−1. The contact time was 6 h.

(mg·g−1) at equilibrium, and qt is the amount of the adsorption (mg·g−1) at any time t. The calculated qe can be obtained from the slope of the kinetics equation. The theoretical qe values are the equilibrium concentrations of heavy metal ions in the adsorbed titania beads assuming 100% of heavy metal ions are removed. Table 1 summarizes the adsorption kinetics parameters of Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions onto the titania beads. The adsorption rates of the tested heavy metal ions followed the tendency of Cu(II) > Co(II) > Cd(II) > Cr(VI) > Cr(III). Case Study on Cr(VI) Ions Removal. We chose the removal of Cr(VI) as the case to study the heavy metal ions removal behavior of mesoporous titania beads in detail. According to the literature,23,28 Cr(VI) ions removal highly depends on the solution pH, and an acidic environment favors their removal. Similar phenomenon was observed in this study (Figure S3, Supporting Information). The Cr(VI) removal

(1) −1

where k2 (g·mg ·min ) is the pseudo-second-order rate constant, qe is the amount of heavy metal ions adsorbed 421

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Table 1. Theoretical and Calculated qe Values, PseudoSecond-Order Rate Constants k2, and Correlation Coefficient Values R2 for Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) Ions Adsorption onto the Titania Beads metal

theoretical qe (mg·g−1)

calculated qe (mg·g−1)

k2 (g·mg−1·min−1)

R2

Cr(VI) Cd(II) Cr(III) Cu(II) Co(II)

10.0 10.0 10.0 10.0 10.0

10.5 9.3 10.4 8.6 8.0

0.0023 0.0086 0.0021 0.0133 0.0087

0.9997 0.9985 0.9885 0.9990 0.9981

efficiencies decreased from 84% to 1% when the pH values increased from 2 to 9. Therefore, the pH values of Cr(VI) initial solutions were adjusted to 2.0 during all the following Cr(VI) removal experiments. The effect of the solution pH on the Cr(VI) ions removal with titania beads could be explained as follows. It is known that the initial pH would govern the surface charge of adsorbent as well as the metal ions. There are plenty of hydroxyl groups on the surface of the titania beads.29,30 When the solution pH is low, titania beads would be protonized to form Ti−OH or Ti−OH2+.29 Since Cr(VI) mainly exists in the form of negatively charged HCrO4− in the acidic solution (pH = 2),31 it could be absorbed by positively charged titania beads through electrostatic attraction. Meanwhile, low pH also facilitates the redox reactions in the aqueous and solid phases, because the protons could participate in the following reaction as eq 232

HCrO4− + 7H+ + 3e ↔ Cr 3 + + 4H2O

(2)

During the adsorption, Cr(VI) was partially reduced to Cr(III) by the reductive surface hydroxyl groups on the titania beads. Figure 4a shows the time profile of Cr(VI) removal with 1.0 g·L−1 of titania beads at different initial Cr(VI) metal ions concentrations at 25 °C. For all concentrations, the adsorption could reach equilibrium within 120 min, similar to the case of Cr(VI) adsorption on Fe@Fe2O3 core−shell nanowires.15 Moreover, the total amount of Cr(VI) adsorbed increased with increasing the initial Cr(VI) concentrations, consistent with other reports.13,15,20,21,23 The maximum Cr(VI) adsorption capacity of the titania beads is found to be 11.5 mg·g−1 when the initial Cr(VI) concentration is 15.0 mg·L−1. The removal efficiencies were in the range of 92% to 77%, decreasing with the increase of initial Cr(VI) concentrations. This is because after consuming all of the higher energy sites, excess Cr(VI) ions would then be adsorbed on the lower energy sites, resulting in loose binding of chromium33 and the decrease of removal efficiency.34 Generally, the initial heavy metal ions concentration is very low in most wastewater, hence high metal ions removal capacity at low concentration is of great importance for the development of adsorbents. The mesoporous titania beads not only possess equal or even higher adsorption capacity compared to Fe@Fe2O3 core−shell nanowires15 and Ce2O,20,21 but also exhibit unsurpassable fast solid−liquid separation property because they could sink down within 4 s (see the video in the Supporting Information). The high adsorption capacity and easy solid−liquid separation property of mesoporous titania beads make them very promising for practical application. The kinetics rates of Cr(VI) removal were further investigated at various initial Cr(VI) concentrations in the presence of 1.0 g·L−1 of titania beads. The kinetics data of sorption of Cr(VI)

Figure 4. (a) Time profile of Cr(VI) ions removal with titania beads. The concentration of mesoporous titania beads was 1.0 g·L−1. The initial Cr(VI) ions concentrations ranged from 2.0 to 15.0 mg·L−1. (b) Removal kinetics curves of Cr(VI) ions ranging from 2.0 to 15.0 mg·L−1. The dosage of titania beads was 1.0 g·L−1. The contact time was 5 h.

ions were modeled using the pseudo-second-order rate equation (eq 1).27 The kinetics curves of the removal process are shown in Figure 4b. Table 2 summarizes the theoretical and calculated qe Table 2. Theoretical and Calculated qe Values, PseudoSecond-Order Rate Constants k2, and Correlation Coefficient Values R2 for Cr(VI) Ions Adsorption onto Titania Beads theoretical qe (mg·g−1)

calculated qe (mg·g−1)

k2 (g·mg−1·min−1)

R2

2.0 4.0 6.0 8.0 10.0 15.0

1.8 3.4 5.5 7.4 10.5 12.7

0.0242 0.0134 0.0057 0.0044 0.0023 0.0019

0.9995 0.9993 0.9988 0.9993 0.9995 0.9992

values, pseudo-second-order rate constants k2 and correlation coefficient values R2. The calculated qe values were found to be very close to the theoretical ones. The plots show considerable good linearity with R2 above 0.998. Therefore, the adsorption kinetics follows the pseudo-second-order model, suggesting a chemisorption process.35 The Cr(VI) adsorption equilibrium was studied by using Langmuir isotherm model and Freundlich isotherm model, respectively. The linear form of the Langmuir model could be 422

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expressed as eq 3.36

1 1 1 1 = · + qe ab Ce b

(3)

The linear form of the Freundlich model could be expressed as eq 4.37

1 log Ce (4) n The adsorption data of Cr(VI) adsorbed onto the titania beads were found to fit better to the Langmuir model (Figure S4a, Supporting Information) with a correlation coefficient R2 value of 0.9211 than to the Freundlich model (Figure S4b, Supporting Information) with a correlation coefficient of 0.8836, indicating the monolayer adsorption of Cr(VI) ions on the surface of mesoporous titania beads. This may be attributed to the homogeneous distribution of surface hydroxyl groups on titania beads and their mesopores ranging from 2 to 6 nm.29,38,39 Regeneration of Saturated Adsorbents. For practical application, the recycling and regeneration of the adsorbent is indispensable. Because of their sizes of several hundred micrometers, the collection of Cr(VI)-adsorbed titania beads was very easy and fast. We treated the Cr(VI)-adsorbed titania beads with NaOH solution and analyzed the concentration of Cr(VI) metal ions desorbed to the solution by atom absorption spectrometry. The desorption efficiency was found to be 86% and there was 0.064 mg of Cr species remained in the titania beads, indicating the adsorbed Cr(VI) ions could be efficiently washed away from titania beads. We used the same batch of mesoporous titania beads to adsorb Cr(VI) for six consecutive cycles (Figure 5) and found that the removal efficiency was log qe = log K +

Figure 6. High-resolution XPS spectra of (a) O 1s of the Cr(VI)adsorbed and fresh titania beads. (b) Cr 2p of the Cr(VI)-adsorbed titania beads.

comparison with that of fresh titania beads (Figure 6a). This comparison shows that Cr(VI) mainly occupies OH surface sites on the titania beads.40 Moreover, the peak of Cr 2p3/2 could be fitted with two peaks at binding energy of ∼577.0 and ∼580.0 eV (Figure 6b), assigned to Cr(III) and Cr(VI), respectively, suggesting that Cr(VI) was partially reduced into Cr(III) by the surface hydroxyl group on titania beads. The resulting Cr(III) was either released back into the solution at lower pH in the form of water-soluble Cr(III) species41 or precipitated on the surface of the titania beads in the form of Cr2O3. With the increase of Cr2O3 remaining on titania beads, the active sites would decrease, causing less Cr(VI) adsorbed by the used titania beads. Meanwhile, the amount of Cr species remaining on the first generated titania beads (0.064 mg) was consistently smaller than that on the third generated ones (0.077 mg), well explaining the reason for the slight removal efficiency decrease during six consecutive cycles of adsorption. It is interesting to find that the structure of mesoporous titania beads could keep well during the recycle experiments without bead cracking and nanoparticle collapsing, even under continuous mechanical agitation with stirring speed of 1000 rpm and stirring time of 30 h. This robust structure of mesoporous titania beads is attributed to their self-bonded nature of the nanoparticles in the beads arisen from the unique alginate biopolymer template method.42 Cr(VI) Ions Removal Mechanism. On the basis of the above results, we proposed a possible Cr(VI) removal

Figure 5. Regeneration studies of mesoporous titania beads with six cycles. The concentration of titania beads was 1.0 g·L−1. The initial Cr(VI) ions concentration was 8.0 mg·L−1. The contact time was 5 h for each cycle.

91% at the first cycle and then slightly deceased to 87% at the sixth cycle. To find out the reason for the slight removal efficiency decrease, X-ray photoelectron spectroscopy was used to analyze the surface chemical compositions of the fresh and Cr(VI)-adsorbed titania beads (Figure 6). It is known that the active sites of TiO2 adsorbent are the surface hydroxyl groups.30 Obviously, the ratio of OH at ∼531.0 eV to O2‑ at ∼529.7 eV in the O 1s spectra of Cr-adsorbed titania beads decreased in 423

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metal ions by selective ion-exchange methods. Environ. Sci. Technol. 2002, 36, 1851−1855. (8) Xing, Y. Q.; Chen, X. M.; Wang, D. H. Electrically regenerated ion exchange for removal and recovery of Cr(VI) from wastewater. Environ. Sci. Technol. 2007, 41, 1439−1443. (9) Juang, R. S.; Shiau, R. C. Metal removal from aqueous solutions using chitosan-enhanced membrane filtration. J. Membr. Sci. 2000, 165, 159−167. (10) Kadirvelu, K.; Faur-Brasquet, C.; Cloirec, P. L. Removal of Cu(II), Pb(II), and Ni(II) by adsorption onto activated carbon cloths. Langmuir 2000, 16, 8404−8409. (11) Fernández-García, M.; Martínez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Nanostructured oxides in chemistry: Characterization and properties. Chem. Rev. 2004, 104, 4063−4104. (12) Mauter, M. S.; Elimelech, M. Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol. 2008, 42, 5843− 5859. (13) Hu, J.; Lo, I. M. C.; Chen, G. H. Fast removal and recovery of Cr(VI) using surface-modified jacobsite (MnFe2O4) nanoparticles. Langmuir 2005, 21, 11173−11179. (14) Feng, Y.; Gong, J. L.; Zeng, G. M.; Niu, Q. Y.; Zhang, H. Y.; Niu, C. G.; Deng, J. H.; Yan, M. Adsorption of Cd(II) and Zn(II) from aqueous solutions using magnetic hydroxyapatite nanoparticles as adsorbents. Chem. Eng. J. 2010, 162, 487−494. (15) Ai, Z. H.; Cheng, Y.; Zhang, L. Z.; Qiu, J. R. Efficient removal of Cr(VI) from aqueous solution with Fe@Fe2O3 core-shell nanowires. Environ. Sci. Technol. 2008, 42, 6955−6960. (16) Liu, X. W.; Hu, Q. Y.; Fang, Z.; Zhang, X. J.; Zhang, B. B. Magnetic chitosan nanocomposites: A useful recyclable tool for heavy metal ion removal. Langmuir 2009, 25, 3−8. (17) Chang, Y. C.; Chen, D. H. Preparation and adsorption properties of monodisperse chitosan-bound Fe 3 O 4 magnetic nanoparticles for removal of Cu(II) ions. J. Colloid Interface Sci. 2005, 283, 446−451. (18) Wang, J. H.; Zheng, S. R.; Shao, Y.; Liu, J. L.; Xu, Z. Y.; Zhu, D. Q. Amino-functionalized Fe3O4@SiO2 core-shell magnetic nanomaterial as a novel adsorbent for aqueous heavy metals removal. J. Colloid Interface Sci. 2010, 349, 293−299. (19) Chen, D. H.; Cao, L.; Huang, F. Z.; Imperia, P.; Cheng, Y. B.; Caruso, R. A. Synthesis of monodisperse mesoporous titania beads with controllable diameter, high surface areas, and variable pore diameters (14−23 nm). J. Am. Chem. Soc. 2010, 132, 4438−4444. (20) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. 3D flowerlike ceria micro/nanocomposite structure and its application for water treatment and CO removal. Chem. Mater. 2007, 19, 1648−1655. (21) Xiao, H. Y.; Ai, Z. H.; Zhang, L. Z. Nonaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres as novel adsorbents for wastewater treatment. J. Phys. Chem. C 2009, 113, 16625−16630. (22) Cao, A. M.; Monnell, J. D.; Matranga, C.; Wu, J. M.; Cao, L. L.; Gao, D. Hierarchical nanostructured copper oxide and its application in arsenic removal. J. Phys. Chem. C 2007, 111, 18624−18628. (23) Zhong, H. X.; Ma, Y. L.; Cao, X. F.; Chen, X. T.; Xue, Z. L. Preparation and characterization of flowerlike Y2(OH)5NO3·1.5H2O and Y2O3 and their efficient removal of Cr(VI) from aqueous solution. J. Phys. Chem. C 2009, 113, 3461−3466. (24) Primo, A.; Marino, T.; Corma, A.; Molinari, R.; García, H. Efficient visible-light photocatalytic water splitting by minute amounts of gold supported on nanoparticulate CeO2 obtained by a biopolymer templating method. J. Am. Chem. Soc. 2011, 133, 6930−6933. (25) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquér ol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, 603−619. (26) Du, K. F.; Yang, D.; Sun, Y. Controlled fabrication of porous titania beads by a sol-gel templating method. Ind. Eng. Chem. Res. 2009, 48, 755−762.

mechanism. When the pH is low, Cr mainly exists in the form of negatively charged HCrO4− and they could be absorbed by positively charged TiO2 beads through electrostatic attraction. During the adsorption via electrostatic attraction, Cr(VI) was partially reduced to Cr(III) by the reductive surface hydroxyl groups on the titania beads. The resulting Cr(III) was either released back into the solution at lower pH in the form of water-soluble Cr(III) species41 or precipitated on the surface of the titania beads in the form of Cr2O3. Obviously, the returning of soluble Cr(III) species into the solution would decrease the overall removal efficiency,41 which well explained why the Cr(VI) removal efficiency of mesoporous titania beads could not reach close to 100% in this study.

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ASSOCIATED CONTENT S Supporting Information * XRD pattern of hydrolyzed product of TTIP; TEM image of ground titania beads; effect of initial pH on the removal of Cr(VI) by titania beads; Langmuir and the Freundlich adsorption isotherms; adsorption rate of Cr(VI) ions by calcium alginate; time profile of Cr(VI) ions removal with titania beads and titania powder ground by titania beads; video of the titania beads sinking. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; phone/fax: +86-27-6786 7535.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program) (Grant 2007CB613301), National Science Foundation of China (Grants 21073069 and 91023010, 21177048), Program for Innovation Team of Hubei Province (Grant 2009CDA048), Self-Determine Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grant CCNU09C01009), and Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT0953).

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dx.doi.org/10.1021/es202043u | Environ. Sci. Technol. 2012, 46, 419−425